Rubber composition for golf ball

ABSTRACT

The invention provides a rubber composition for golf balls which includes (A) a base rubber containing a polybutadiene having a cis-1,4 bond content of at least 60 wt %, (B) an unsaturated carboxylic acid and/or a metal salt thereof, and (C) a benzimidazole of general formula (1) or (2) below 
                         
wherein R is a hydrogen atom or a hydrocarbon group of 1 to 20 carbons, and m is an integer from 1 to 4, with the proviso that if m is 2 or more, each occurrence of R may be the same or different.
 
     The golf ball rubber composition of the invention enables a high-quality molded and crosslinked product having a suitable hardness and a high resilience to be obtained.

BACKGROUND OF THE INVENTION

The present invention relates to a rubber composition for use inone-piece golf balls and, for example, the cores of solid golf ballssuch as two-piece golf balls and multi-piece golf balls. Morespecifically, the invention relates to a rubber composition which, in amolded and crosslinked form, has a suitable hardness and a goodresilience, and is thus ideal as a golf ball material.

One-piece golf balls, and the solid cores of two-piece golf balls andmulti-piece golf balls which are composed of a solid core encased,either directly or over an intervening intermediate layer, by a cover,are generally obtained by vulcanizing a rubber composition containing asthe base material a rubber component such as polybutadiene andcontaining also, for example, an unsaturated carboxylic acid metal saltsuch as zinc acrylate and an organic peroxide.

The unsaturated carboxylic acid metal salt serves primarily as aco-crosslinking agent or a crosslinking aid in the rubber composition,and is known to have a large influence on the crosslink structure andcrosslink density of the rubber.

Peroxide crosslinking is employed to crosslink rubber, and is carriedout using one or more organic peroxide. The field of golf balls containsmuch recent prior art that employs two or more organic peroxides bymaking use of the difference in the decomposition temperatures of theorganic peroxides. A variety of research exists on such peroxidecrosslinking.

In addition to these organic peroxides and unsaturated carboxylic acidmetal salts, various types of additives, such as antioxidants, sulfur,organosulfur compounds, inert fillers and zinc stearate, are alsosuitably included in such rubber compositions for golf balls in order toimprove the physical properties and characteristics thereof.

However, golf ball users are highly demanding and so furtherimprovements in performance are desired. There is an especially strongdesire for improvements in rebound, which directly affects the flightperformance.

Prior-art references which relate to the present invention include thefollowing.

Patent Document 1: JP-A 2002-355339

Patent Document 2: JP-A 2002-003653

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide, basedupon further investigations on various additives which are included inrubber compositions, a rubber composition for golf balls which increasesthe resilience of the composition when molded and crosslinked and alsoenables a molded and crosslinked product of suitable hardness to beobtained.

The inventors have conducted intensive investigations, as a result ofwhich they have discovered that, in the preparation of a rubbercomposition for a one-piece solid golf ball or for a core or portion ofa solid golf ball having a cover of one, two or more layers, when aspecific polybutadiene-containing base rubber is compounded with anunsaturated carboxylic acid and/or a metal salt thereof and a molded andcrosslinked rubber material is obtained therefrom using a crosslinkingagent such as an organic peroxide, by including in the rubbercomposition a benzimidazole of general formula (1) or (2) below.

(wherein R is a hydrogen atom or a hydrocarbon group of 1 to 20 carbons,and m is an integer from 1 to 4, with the proviso that if m is 2 ormore, each occurrence of R may be the same or different), thebenzimidazole increases the resilience of the molded and crosslinkedmaterial and also enables the material to maintain a suitable hardness.)

Accordingly, the invention provides the following rubber composition forgolf balls.

-   [1] A rubber composition for golf balls, comprising the following    components A to C:

(A) a base rubber containing a polybutadiene having a cis-1,4 bondcontent of at least 60 wt %,

(B) an unsaturated carboxylic acid and/or a metal salt thereof, and

(C) a benzimidazole of general formula (1) or (2) above.

-   [2] The rubber composition for golf balls of [1], wherein the    benzimidazole serving as component C is 2-mercaptobenzimidazole.-   [3] The rubber composition for golf balls of [1], wherein the    benzimidazole serving as component C is included in an amount of    from 0.01 to 5 parts by weight per 100 parts by weight of component    A.-   [4] The rubber composition for golf balls of [1] which further    comprises, as a crosslinking agent, (D) an organic peroxide.-   [5] The rubber composition for golf balls of [1] which, in a molded    and crosslinked form, is adapted for use as a core.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

This invention provides a rubber composition obtained by compounding (A)a base rubber, (B) an unsaturated carboxylic acid and/or a metal saltthereof, and (C) a benzimidazole of formula (1) or (2) above. Theformulation of the rubber composition is described in detail below.

Preferred use may be made of polybutadiene as the base rubber serving ascomponent A. In particular, it is recommended that use be made of apolybutadiene having a cis-1,4 bond content on the polymer chain of atleast 60 wt %, preferably at least 80 wt %, more preferably at least 90wt %, and most preferably at least 95 wt %. If the content of cis-1,4bonds among the bonds on the molecule is too low, the resilience maydecrease.

The content of 1,2-vinyl bonds on the polybutadiene is preferably notmore than 2%, more preferably not more than 1.7%, and even morepreferably not more than 1.5%, of the bonds on the polymer chain. If thecontent of 1,2-vinyl bonds is too high, the resilience may decrease.

Rubber ingredients other than the above polybutadiene may be included inabove component A within a range that is not detrimental to theadvantageous effects of the invention. Examples of such rubberingredients other than the above-described polybutadiene include otherpolybutadienes, and other diene rubbers, such as styrene-butadienerubber, natural rubber, isoprene rubber and ethylene-propylene-dienerubber.

The unsaturated carboxylic acid and unsaturated carboxylic acid metalsalt serving as component B is included as a co-crosslinking agent.

Examples of the unsaturated carboxylic acid include, but are not limitedto, acrylic acid, methacrylic acid, maleic acid and fumaric acid. Theuse of acrylic acid and methacrylic acid is especially preferred.

The unsaturated carboxylic acid metal salt is exemplified by, but notlimited to, the above unsaturated carboxylic acids neutralized withdesired metal ions. Illustrative examples include the zinc salts andmagnesium salts of methacrylic acid and acrylic acid. Zinc acrylate isespecially preferred.

The amount of component B included per 100 parts by weight of the baserubber may be set to preferably at least 10 parts by weight, and morepreferably at least 15 parts by weight. The upper limit in the amountincluded per 100 parts by weight of the base rubber may be set topreferably not more than 60 parts by weight, and more preferably notmore than 45 parts by weight. If too much is included, the ball maybecome too hard, which may result in an unpleasant feel on impact. Onthe other hand, it too little is included, the rebound may decrease.

The benzimidazole serving as component C is a compound of generalformula (1) or (2) below

In the formula, R is a hydrogen atom or a hydrocarbon group having 1 to20 carbons, and m is an integer from 1 to 4, with the proviso that if mis 2 or more, each occurrence of R may be the same or different.

Here, R in above formula (1) and (2) is a hydrogen atom or a hydrocarbongroup having from 1 to 20 carbons. The hydrocarbon group is not subjectto any particular limitation, although methyl and ethyl are especiallypreferred. Some or all of the hydrogen atoms on the hydrocarbon groupmay be substituted, the substituents being exemplified by halogen atomsand hydroxyl groups. Moreover, some or all occurrences of R in formula(1), (2) may be like or unlike.

Illustrative examples of the benzimidazole represented by above generalformula (1), (2) include 2-mercaptobenzimidazole,

-   2-mercaptomethylbenzimidazole,-   2-mercaptodimethylbenzimidazole,-   2-mercaptotrimethylbenzimidazole,-   2-mercaptotetramethylbenzimidazole, 2-benzimidazolethione and-   5-methyl-2-benzimidazolethione. Of these, the use of-   2-mercaptobenzimidazole (formula (3) below) or-   2-mercaptomethylbenzimidazole (formula (4) below) is preferred.

The amount of benzimidazole included as component C in the rubbercomposition has a lower limit of preferably 0.01 part by weight, andmore preferably 0.02 part by weight, per 100 parts by weight ofcomponent A. The upper limit is preferably 5 parts by weight, morepreferably 4 parts by weight, and even more preferably 2 parts byweight, per 100 parts by weight of component A. If too little componentC is included, the intended effects of improving rebound and increasingthe initial velocity may not be obtained. On the other hand, if too muchis included, the hardness may become too low, which may result in toolarge a deflection or make it impossible to achieve a rebound-improvingeffect.

A known crosslinking agent is included in the inventive rubbercomposition. Although not subject to any particular limitation, in thepresent invention, it is preferable to use for this purpose an organicperoxide (component D). Known organic peroxides may be used as thisorganic peroxide. Illustrative examples include dicumyl peroxide,1,1-di(t-butylperoxy)cyclohexane, dibenzoyl peroxide, dilauroyl peroxideand 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane. These organicperoxides may be used singly or as combinations of two or more thereof.Commercial products may be used as the organic peroxide. Illustrativeexamples of such commercial products include those available under thetrade names “Percumyl D” and “Perhexa C-40” (both from NOF Corporation),the trade names “Niper BW” and “Peroyl L” (both from NOF Corporation),and the trade name “Trigonox 29” (from Kayaku Akzo Corporation).

The amount of organic peroxide (component D) included is suitably setaccording to, for example, the type of organic peroxide and the moldingand crosslinking conditions that are selected. Although not subject toany particular limitation, the amount included per 100 parts by weightof component A is preferably at least 0.01 part by weight, morepreferably at least 0.05 part by weight, and even more preferably atleast 0.1 part by weight. The upper limit is preferably not more than 15parts by weight, more preferably not more than 10 parts by weight, andeven more preferably not more than 5 parts by weight. If too littleorganic peroxide is included as component D, a sufficientrebound-improving effect may not be obtained. On the other hand, if toomuch is included, a further rebound-improving effect (especially whenthe ball is struck with a W#1) cannot be expected, in addition to whichthe ball may become too soft or the feel on impact may worsen.

In addition to above components A to D, the rubber composition of theinvention may optionally include also various additives. For example,sulfur, organosulfur compounds, inert fillers, antioxidants and zincstearate may be included.

As inert fillers, preferred use may be made of, for example, zinc oxide,barium sulfate or calcium carbonate. These may be used singly or ascombinations of two or more thereof.

The amount of inert filler included per 100 parts by weight of the baserubber may be set to preferably at least 1 part by weight, and morepreferably at least 5 parts by weight. The upper limit in the amount ofinert filler per 100 parts by weight of the base rubber may be set topreferably not more than 200 parts by weight, more preferably not morethan 150 parts by weight, and even more preferably not more than 100parts by weight. Too much or too little inert filler may make itimpossible to achieve a proper weight and a good rebound.

The antioxidant used may be a known antioxidant.

Illustrative, non-limiting, examples include the commercial productsNocrac NS-6, Nocrac NS-30, Nocrac SP-N and Nocrac 200 (all availablefrom Ouchi Shinko Chemical Industry Co., Ltd.). These may be used singlyor as a combination of two or more thereof.

The amount of antioxidant included per 100 parts by weight of the baserubber serving as component A is set to preferably not more than 5 partsby weight, and more preferably not more than 3 parts by weight.Including too much may make a good rebound and a good durabilityimpossible to achieve.

The rubber composition of the invention forms at least some portion ofan integral part of a golf ball, which golf ball may take any of variousforms according to the intended purpose thereof. Illustrative examplesinclude a one-piece golf ball formed entirely of the above rubbercomposition; a two-piece solid golf ball having a solid core and onecover layer, wherein at least some portion of the solid core and/or thecover is formed of the above composition; a multi-piece solid golf ballhaving a solid core of one, two or more layers and a cover of one, twoor more layers, wherein at least some portion of the solid core and/orthe cover is formed of the above composition; and a wound golf ballwherein at least some portion of the solid center and/or the cover isformed of the above composition. Of these various forms, particularlyfrom the standpoint of taking full advantage of the properties of thecomposition and more effectively achieving a good golf ball rebound, atwo-piece solid golf ball or a multi-piece solid golf ball having asolid core formed of the above composition is preferred.

When constructing a golf ball, examples of materials that may be used inportions of the golf ball other than the portion where the inventiverubber composition for golf balls is used include thermoplastic orthermoset polyurethane elastomers, polyester elastomers, ionomer resins,polyolefin elastomers and polyureas. One of these types may be usedalone or two or more types may be used in admixture. The use of athermoplastic polyurethane elastomer or an ionomer resin is especiallypreferred. The molding process used may be a known process such asinjection molding or compression molding.

Commercial products may be used as the thermoplastic polyurethaneelastomer. Illustrative examples include Pandex T7298, Pandex T7295,Pandex T7890, Pandex TR3080, Pandex

T8295, Pandex T8290 and Pandex T8260 (available from DIC Bayer Polymer,Ltd.). Commercial products may be used as the ionomer resin as well.Illustrative examples include Surlyn 6320, Surlyn 8120 and Surlyn 9945(available from E.I. DuPont de Nemours & Co.), and Himilan 1706, Himilan1605, Himilan 1855, Himilan 1601 and Himilan 1557 (available fromDuPont-Mitsui Polychemicals Co., Ltd.).

Polymers such as thermoplastic elastomers other than those mentionedabove may be included as optional ingredients in the above material.Examples of such other polymers include polyamide elastomers, styreneblock elastomers, hydrogenated polybutadienes and ethylene-vinyl acetate(EVA) copolymers.

In cases where the golf ball manufactured using the rubber compositionof the invention is a one-piece golf ball or a golf ball having a solidcore or a solid center, the deflection of the one-piece golf ball or thesolid core or solid center, when compressed under a final load of 1,275N (130 kgf) from an initial load state of 98 N (10 kgf), although notsubject to any particular limitation, is generally at least 2.0 mm, andpreferably at least 2.5 mm. The upper limit is generally not more than6.0 mm, and preferably not more than 5.8 mm. If the deflection is toosmall, the feel on impact may worsen and, particularly on a long shotsuch as with a driver that causes a large ball deformation, the spinrate may rise excessively, resulting in a poor distance. On the otherhand, if the one-piece golf ball or the solid core or solid center istoo soft, the ball may have a deadened feel on impact and the reboundmay be inadequate, possibly resulting in a poor distance, or thedurability of the ball to cracking on repeated impact may worsen.

Here, the diameter of the solid core, although not subject to anyparticular limitation, may be set to generally at least 20 mm, andpreferably at least 30 mm. The upper limit may be set to generally notmore than 42.5 mm, and preferably not more than 42.3 mm.

The specific gravity of the solid core, although not subject to anyparticular limitation, may be set to generally at least 0.7, andpreferably at least 0.9. The upper limit may be set to generally notmore than 1.6, and preferably not more than 1.4.

The thickness per cover layer formed of the inventive golf ballcomposition and the above-mentioned materials, although not subject toany particular limitation, may be set to generally at least 0.1 mm, andpreferably at least 0.2 mm. The upper limit may be set to generally notmore than 4 mm, and preferably not more than 3 mm.

In cases where the rubber composition of the invention is used toproduce one-piece golf balls, two-piece solid golf balls and multi-piecesolid golf balls, production may be carried out by effectingvulcanization and curing in the same way as with conventional rubbercompositions for golf balls. The vulcanization conditions areexemplified by a vulcanization temperature of from 100 to 200° C. and avulcanization time of from 10 to 40 minutes.

When a golf ball is manufactured using the rubber composition of theinvention, the diameter of the ball may be set to not less than 42 mm,and in particular, according to the Rules of Golf for competitive play,preferably not less than 42.67 mm. The upper limit may be set to notmore than 45 mm, and preferably not more than 44 mm. The weight may beset to not more than 48 g, and in particular, according to the Rules ofGolf for competitive play, preferably not more than 45.93 g. The lowerlimit may be set to not less than 40 g, and preferably not less than 44g.

As described above, the rubber composition for golf balls of thisinvention enables a high-quality molded and crosslinked product having asuitable hardness and a high resilience to be obtained. In particular,by employing this rubber composition as the material for a one-piecegolf ball or as the solid core material in a multilayer solid golf ball,a high initial velocity, an increased distance, and a good feel onimpact can be obtained.

EXAMPLES

Examples of the invention and Comparative Examples are given below byway of illustration, and not by way of limitation.

Examples 1 to 6, Comparative Example 1

Formation of Core

Rubber compositions were formulated as shown in Table 1, then molded andvulcanized at 155° for 20 minutes to form cores having the diameters andweights shown in the table. Ingredient amounts in the table areindicated in parts by weight.

TABLE 1 Comparative Example Example 1 1 2 3 4 5 6 BR730 100 100 100 100100 100 100 Zinc oxide 22 22 22 22 22 22 22 Zinc acrylate 30 30 30 30 3030 30 Nocrac NS-6 0.1 Nocrac MB 0.02 0.04 0.30 0.44 0.50 1.00 Percumyl D0.3 0.3 0.3 0.3 0.3 0.3 0.3 Perhexa C-40 0.3 0.3 0.3 0.3 0.3 0.3 0.3Diameter, mm 38.4 38.5 38.6 38.6 38.5 38.6 38.5 Weight, g 35.4 35.4 35.635.6 35.5 35.5 35.6 Deflection, mm 3.11 2.86 2.69 2.89 2.61 2.93 2.76Initial velocity, m/s — +0.41 +0.26 +0.24 +0.27 +0.20 +0.17

Details on the materials in Table 1 are given below.

-   BR730: A polybutadiene available from JSR Corporation; cis-1,4 bond    content, 95% (published value from JSR Corporation)-   Zinc oxide: Available from Sakai Chemical Co., Ltd.-   Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd.-   Nocrac NS-6: An antioxidant available from Ouchi Shinko Chemical    Industry Co., Ltd.; 2,2′-methylenebis(4-methyl-6-t-butylphenol)-   Nocrac MB: 2-Mercaptobenzimidazole, available from Tokyo Chemical    Industry Co., Ltd.-   Percumyl D: An organic peroxide available from NOF Corporation;    dicumyl peroxide-   Perhexa C-40: An organic peroxide available from NOF Corporation;    1,1-di(t-butylperoxy)cyclohexane, 40% concentration

The deflections and initial velocities of the respective cores obtainedwere evaluated by the following methods. The results are shown in Table1.

(1) Core Deflection (mm)

The deflection (mm) of the core when compressed at a temperature of23±1° C. and a rate of 10 mm/s under a final load of 1,275 N (130 kgf)from an initial load state of 98 N (10 kgf) was measured.

(2) Core Initial Velocity Test (m/s)

The initial velocity of the core was measured using an initial velocitymeasuring apparatus of the same type as the USGA drum rotation-typeinitial velocity instrument approved by the R&A. The core was heldisothermally at a temperature of 23±1° C. for at least 3 hours, thentested in a room temperature (23±2° C.) chamber. The differencesrelative to the core in Comparative Example 1 are shown in Table 1.

The results shown in Table 1 confirmed that the molded and crosslinkedproducts (cores) obtained from the 2-mercaptobenzimidazole-containingrubber compositions of Examples 1 to 6 exhibited a higher resilience andwere capable of achieving a higher core initial velocity than the rubbercomposition of Comparative Example 1, which did not contain2-mercaptobenzimidazole.

the invention claimed is:
 1. A rubber composition for golf balls,comprising the following components A to C: (A) a base rubber containinga polybutadiene having a cis-1,4 bond content of at least 60 wt %, (B)an unsaturated carboxylic acid and/or a metal salt thereof, and (C)2-mercaptobenzimidazole in an amount of from 0.01 to 5 parts by weightper 100 parts by weight of component A.
 2. The rubber composition forgolf balls of claim 1 which further comprises, as a crosslinking agent,(D) an organic peroxide.
 3. The rubber composition for golf balls ofclaim 1 which, in a molded and crosslinked form, is adapted for use as acore.